1. Field of the Invention
This invention involves generally devices for sensing and effecting force and pressure and more particularly involves a pressure transducer that reversibly converts mechanical force or pressure into an electrical voltage by means of an electrochemical process.
2. Description of the Related Art
Several methods for measuring force and pressure are known in the art. Except for gravity column measurements, such methods generally use a pressure transducer to convert applied pressure to an electrical voltage. Pressure transducers generally rely on an intermediate physical displacement induced by the applied pressure to generate an electrical voltage. The transduction principles or means of transforming a change in physical displacement into a change in electrical voltage or current circuit are as varied as the many physical relationships known to modern science. However, the transduction techniques in general use are limited to a relative few. These are essentially those which involve variations in resistance, inductance, and reluctance. Useful data are obtained by supplying such transducers with an input voltage and developing an output voltage as a function of the changes in displacement produced as an intermediate result of the application of the pressure or force to be measured.
Of the remaining transduction principles, the piezoelectric and vibrating-wire types are most commonly used. The piezoelectric transducer uses a crystalline material that produces an output voltage proportional to the rate of change in strain over time. Because the output voltage is generated only under dynamic conditions, the piezoelectric device cannot be used for static or steady-state pressure or force measurements. It is normally used as a miniature instrument intended to measure vibration or dynamic pressure.
The vibrating-wire transducer comprises a fine wire supported so that its tension and strain varies according to the applied pressure or force. When this wire lies between the poles of a permanent magnet, it vibrates at its natural frequency upon application of an oscillating voltage. The vibrating-wire transducer output is an oscillating voltage that changes in frequency according to changes in the wire tension induced by the applied pressure or force.
Each of these transducer principles requires a displacement to generate a voltage output from the sensor. Pressure measurements are achieved by summing applied pressure over a flexible material that in turn is permitted limited travel. The flexible member may be a diaphragm, capsule, bellows or several variations of bourdon tubes. Each of these provides a limited displacement under applied pressure. Absolute pressures are generally determined by sealing either the capsule or bellows assembly, or the instrument chamber, at a reference pressure usually close to zero. Gauge pressure (relative to ambient atmospheric pressure) is generally measured by connecting the reference side of the instrument to the ambient condition. Differential pressure can be measured by permitting the two pressures in question access to opposite sides of the sensing member.
Each of the above-mentioned transduction techniques is subject to several limitations. Except for the piezoelectric transducer, all such methods require a separate voltage source. The piezoelectric transducer requires no separate voltage source, but it is not useful for static pressure measurement. Again except for the piezoelectric transducer, none of the above transduction techniques are amenable to self-calibration. Although a piezoelectric transducer can be self-calibrated by means of the reciprocity principle, this method is useful only for dynamic (sinusoidally-varying) pressures. A simple, self-calibrating pressure sensor that can be calibrated to accurately sense the absolute value of static pressure without reference to a calibration standard is unknown in the art.
In addition to the simple measurement of force or pressure, two other related sensing requirements exist in the art. First, there is often a requirement for measurement of the partial-pressure of a gas in a mixture of gases. For instance, the direct sensing of the partial-pressure of oxygen (O.sub.2) is required in the arts of medicine, biology, automotive engineering, chemistry, petroleum engineering, and others. Secondly, in the emerging robotics arts there is a well-known requirement for a transducer that combines the capacity to sense force or pressure with the capacity to effect a desired force or pressure. Such a device may be considered a force or pressure sensor/effector and is useful for robotic manipulators. The above-mentioned force transduction techniques cannot meet either of these important requirements because the transduction phenomenon is either nonreversible or (in the case of the piezoelectric transducer) the transduction phenomenon does not exist under static conditions.
Another method for measuring the partial pressure of oxygen was demonstrated by R. Richter at Jet Propulsion Laboratory in 1984, as reported in Richter, R., "Measuring Absolute Oxygen Pressure," NASA Tech Brief 8, No 3, Item #97. Richter did his work with a high temperature (&gt;1000.degree. C.) zirconia-electrolyte fuel cell, using it to measure the partial-pressure of oxygen. Richter's method is not useful for measuring the partial-pressure of other electrochemically active gases (eg. H.sub.2, Cl.sub.2, et al) at ambient temperature.
An electrochemical cell is typically formed by positioning an electrolytic membrane between and in contact with a cathode and an anode. Although such a cell can either generate electricity (as a fuel cell) or do mechanical work (as a motor), it has not heretofore been used as a transducer for the sensing or effecting of force and pressure. When the cell is configured as a fuel cell to generate electricity, a fuel gas such as hydrogen is supplied to the anode and a gaseous oxidant such as oxygen is supplied to the cathode, as disclosed by Wentworth in U.S. Pat. No. 3,418,168. When the cell is configured as a motor to produce mechanical energy, an electrical voltage is applied across the anode and cathode and an electrochemically active gas capable of entering into an oxidation/reduction reaction is supplied to the anode, as disclosed in my U.S. Pat. No. 4,402,817. Neither of these patents teaches or suggests the use of an electrochemical cell as a force or pressure sensor. Such an application is briefly disclosed by in my copending patent application U.S. Ser. No. 07/563,050 entitled "Efficient Electrochemical Motor" filed concurrently herewith.